Java based learning (e): String class, common string operations

Java based learning (e): String class, common string operations

A, String class, String

1. String is constant, its value can not be changed after creation, not a variable length.

String str = "hahaha" ;

2. If you do not initialize a string object, a variable does not point to any memory address, then calls will go wrong.
3. It can be used to create an empty string null assignment.

String str = null ;

4. Initialization string
   · String str = "hahaha"
   · String str = new String ("hahaha")
   · string str = String.valueOf(其他任意类型)(used to any other type of elementary data type is converted into a string)
5. With a character array initialization
   * new String(char[])
   * new String(char[],int,int)(int, int character interception array of characters among the int1 to int2)
   · String.valueOf(char[])
   ·String.valueOf(char[],int,int)

public class first {
	public static void main(String[] args) {
			char[] ch = {'h','a','h','a'};
			String s1 = new String(ch);
			String s2 = new  String(ch,1,2);
			String s3 = String.valueOf(ch);
			String s4 = String.valueOf(ch,1,2);
			System.out.println(s1+"\n"+s2+"\n"+s3+"\n"+s4);
	}
}
//-------------输出---------------
haha
ah
haha
ah

Second, the common string manipulation: Stitching

1. · "+" Plus sign is used to splice a plurality of strings, a string or must be noted that both sides of the String object. (Right operand is forcibly converted into String type).
   · "+ =" That is str += str1 => str = str + str1.
   · 字符串.concat（字符串） (Returns a new object, the object itself does not change the original, you must do the assignment operator).
2. Character length:字符串.length()

public class first {
	public static void main(String[] args) {
			char[] ch = {'h','a','h','a'};
			String s1 = new String(ch);
			System.out.println(s1.length());
	}
}

Third, the common string operations: the index of the string

1. the charAt () Method:
   Use charAt(int index)method to obtain a character string specified index, int value for the parameter into the method, a first string index value 0, the character is the last character index length minus 1, returns to a char type.

public class first {
	public static void main(String[] args) {
			String s1 = "hahehrht";
			System.out.println(s1.charAt(1));
	}
}
//-----------输出-------------
a

2. the indexOf () method:
   using the indexOf () method may return into the reference characters in the specified String object first occurrence index position, -1 on failure.
   indexOf () Overloaded there are four:

method	Functional Description
indexOf(int ch)	Returns the index into the reference position of the first occurrence of the character in the specified String object, into the reference to Unicode character encoding
indexOf(int ch , int fromIndex)	fromIndex start the search from the reference index, the index returned into the position reference character first appeared in the specified String object, into the reference to Unicode character encoding
indexOf(String str)	Returns the index position of the first reference string appears in the specified String object
indexOf(String str , int fromIdex)	Search from the reference index fromIndex, returns the index into the reference position of the first occurrence of the string in the specified String object

Fourth, the common string manipulation: the replacement string

method	Functional Description
replace(char old , char new)	Replace the old new, note the string old will all be replaced by new, support for regular expressions
replaceAll(String regex , String replacement)	From the top of the string to start the search until the tail, are matched to the characters, strings, regular expressions will be replaced
replaceFirst(String regex , String replacement)	Replace begin the search string from the top until the tail, are matched to the characters, strings, regular expressions, only the first match

Five common string operation: string interception

method	Functional Description
substring(int beginIdex)	Start from the parameters taken index position, until the end of the specified String object
substring(int beginIdex ， int endindex)	The reference index is intercepted from the start position, the position of the end endindex-1 (left and right open and closed)

Common operating six string: string split

method	Functional Description
split(String regex)	The string into a string in accordance with a certain plan of an array can be "," delimiters may be a regular expression

String[] str = str.split(",");

Seven common string operations: determine the contents of the string end to end, end to end to the string of blanks, the string case conversion

operating	method	Functional Description
Analyzing the content of the string end to end:	startsWith(String prefix)	Determining whether the character string beginning with a prefix
	startsWith(String prfix , int fromidex)	Determining whether the string index from the beginning with a prefix
	endsWith(String suffix)	Determining whether the suffix string begins
String end to end to space:	trim()	Returns a string string trailing spaces removed
String case conversion:	toLowerCase()	Each character in the specified object is converted into lowercase
	toUpperCase()	Each character in the given object is converted to uppercase

Eight common string manipulation: string comparison
in Java, whether the use of "==" to compare the data are equal, and compare strings in double equal sign is a reference to the same memory address.

method	Functional Description
equals(String anotherString)	Comparison string of
equealsIgnoreCase(String anotherString)	Compare strings, case-insensitive

Common operating IX strings: other characters operations

method	Functional Description
contains(CharSequences)	Determining whether to include a String object into the reference string, it returns a Boolean value
compareTo(String antherString)	In accordance with the difference code comparing ASCLL, then continue to the next if the same comparison, if the different return ASCLL
hashCode()	Returns a hash value of the string
toCharArray()	The method may be converted into a string of new character array
toString()	Converted to a string

Ten, formatted output

format(string str , Objetc...args)

The conversion operation	Explanation
%s	String type
%c	Character Types
%b	Boolean
%d	Integer type (decimal)
%x	Integer type (hexadecimal)
%The	Integer type (octal)
%f	Floating-point type
%a	Hexadecimal floating-point type
%e	Index type
%g	General floating-point type
%%	Percentage Type
%n	Newline
%tx	Time as Date Type

E.g:

public class first {
	public static void main(String[] args) {
			String s1 = "hahehrht";
			System.out.println(String.format("%s这是一个字符串",s1));
	}
}
//--------------输出-------------
hahehrht这是一个字符串

format also provides time formatting operations: link